Camera tube target including porous photoconductive layer comprising antimony trisulfide, free antimony and copper phthalocyanine



March 5, 1968 J.. CAMERA TUBE. TARGET INCLU K FISHER DING POROUSPHOTOCONDUCTIVE LAYER COMPRISING ANTIMONY TRISULFIDE, FREE ANTIMONY ANDCOPPER PHTHALOCYANINE Filed July 29, 1966 V, r I z I I l f I I z 1 l l II N VEN TOR.

United States Patent 3,372,294 CAMERA TUBE TARGET INCLUDING POROUSPHOTOCONDUCTIVE LAYER COMPRISING ANTIMONY TRISULFIDE, FREE ANTIMONY ANDCOPPER PHTHALOCYANINE James K. Fisher, Garland, Tex., assignor toGeneral Electrodynamics Corporation, Garland, Tex., a corporation ofTexas Continuation-impart of application Ser. No. 452,640, May 3, 1965.This application July 29, 1966, Ser. No. 568,823

1 Claim. (Cl. 31365) ABSTRACT OF THE DISCLOSURE A television camera tubehaving a photosensitive surface containing an inorganic photoconductorand a minor proportion of an organic photoconductor, such as copperphthalocyanine.

This is a continuation-in-part of application Ser. No. 452,640, filedMay 3, 1965.

This invention relates to electron discharge devices, and moreparticularly it relates to photoconductive targets for use in electrondischarge devices, especially of the vidicon television camera tubetype.

Vidicon tubes are well known in the television industry inasmuch as suchvidicon tubes are in wide use, particularly in televising motionpictures. The general construction and operation of vidicon tubes aredescribed on pages 2222 and 22-23 of Radio Engineering Handbook, FifthEdition, published by McGraw-Hill Book Company, Inc. A vidicon tube hasa photoconductive coating applied to a support which is transparent tothe radiation to be viewed, and which is made photoconductive by theimposition of a radiation image thereon. Electrons from an electron gunimpinge upon the coating and create a signal which becomes the picturesignal. It is well known that the functioning and operation of vidicontubes can be greatly improved by close attention to the photoconductivecoatings utilized in the targets.

As is well known in the art, photoconcluctive materials havecomparatively high resistivities, but become conductive upon exposure tolight. The sensitivity of such materials with various wave lengths oflight is measured in microamps of current per unit of radiant energyimpinging thereon. A coating of good sensitivity may have a sensitivityas high as from about 150 to 400 microamps per lumen.

A good vidicon target must also have a lag, or decay time, short enoughfor signals to be erased between scannings by the electron gun. In otherwords, the lag, or decay time, must be shorter than the time intervalbetween two successive scannings of the same image element on thephotoconductive coating. For the usual commercial television operationthe lag should be short enough that the vidicon is capable ofreproducing pictures of fast moving objects without a shadow or lighttrail. This requires a lag of something less than of a second.

Stanley V. Forgue has described, in his US. Patent 2,967,254, thepreparation of a target for a photoconductive camera tube whichcomprises a conductive coating comprising a signal electrode applied toa light transparent window, a layer of a porous inorganicphotoconductive material such as antimony trisulfide applied over thesignal electrode, and a solid layer of an inorganic photoconductiv-ematerial such as antimony trisulfide applied over the porous layer. Asdisclosed by Forgue, a porous layer is obtained by evaporating thematerial in a gaseous atmosphere at a comparatively poor vacuum of fromabout 2 1O- to several mm. of mercury, while the solid layer of materialis deposited in a good vacuum of the order of 10- mm.

A major problem encountered in the use of vidicon tubes is known in theindustry as white burn-in. The phenomenon of white burn-in constitutesthe forming of a negative after-image on the target of the vidicon. Forexample, in situations where the camera tube views a stationary scenefor an extended period of time a negative image of this stationary senemay remain on the target and will show up on television display tubes.The reason for this white burn-in is not known, but it has been notedthat modifications of photoconductive coatings to improve theirsensitivities has resulted in a decrease of resistance to white burn-in.Thus, manufacturers of vidicon tubes have been unable to utilizephotoconductive coatings of the highest sensitivities because of thisobjectionable feature.

The present invention, however, provides a photoconductive coating thathas unusually good sensitivity with low lag but with less white burn-inthan had heretofore been obtainable with the high sensitivity coating.

The superior performance of the photosensitive device of this inventionis attained by using, with the usual inorganic photoconductor, a smallproportion of .an organic photoconductive material. It has been foundthat the use of such a minor proportion of organic photoconductortogether with the usual inorganic photoconductors im proves thesensitivity of the camera tube and decreases the lag time whilesubstantially increasing the resistance to white burn-in.

Reference is now made to the accompanying drawing wherein the singlefigure portrays a vidicon camera tube with the target end shown incross-section.

As shown in the drawing, the envelope 10 of the camera tube has attachedthereto a metal ring 12 sealed to the end of the tube, the metal ringbeing made of a material such as Kovar. The end of the tube is closed bya light transparent face plate 15 which maybe made of glass or quartz orthe like, and which comprises a support for the target materials. In theembodiment shown in the drawing the target comprises a first conductivelayer 16, a second porous or spongy layer 18, and a third solid vacuumdeposited layer 20.

The layers are preferably deposited by evaporation by means well knownin the art and as explained for example, in the aforesaid patent toForgue. As is also well known in the art, the conductive layer 16 may beany one of a number of known conductive coating materials such as forexample, tin oxide or tin fluoride.

According to one embodiment of this invention, in preparing the spongyor porous layer 18 a mixture of organic and inorganic materials isevaporated in a poor vacuum onto the surface. One preferred mixturecomprises antimony trisulfide; free antimony, and copper phthalocyanine,the free antimony in the mixture being about 10% of the weight of theantimony trisulfide, and the amount of copper phthalocyanine being about5% of the antimony trisulfide-free antimony mixture. Following thedeposition of this mixture in a spongy coating 18 on the signal platecoating 16, a solid coating of antimony trisulfide, preferably continingabout 15 parts by weight of free antimony per parts of antimonytrisulfide, is then evaporated onto the surface. Thereafter, the targetis made more sensitive by heating, as is known in the art.

Although it is preferred to co-evaporate the organic and inorganicmaterials, substantially improved results in accordance with thisinvention can be obtained by separately evaporating the organic andinorganic photoconductors so that one of them forms a coating before theother one is evaporated onto the signal plate. Only a minor amount ofthe organic material is used, usually no more than about 4% to about 10%of the weight of the inorganic photoconductor, so that the organiccoating I such as aluminum phthalocyanine and chromium phthalocyanine,give good results in the composition of this invention. In addition,other organic photoconductors known in the art may be used to accomplishthe unexpected results of this invention to a greater or lesser degree.Fused ring dye materials, such as anthracene and phenanthrene and otherorganornetallic dyes, are included within such organic photoconductors.Preferably, materials are used which have a comparatively low vaporpressure.

It is noted that the use of the organic material combined with theinorganic photoconductive material substantially improves sensitivityand lag characteristics of the coating. The improved sensitivity isparticularly significant in the red end of the visible spectrum.Furthermore, White burn-in is much less of a problem than with ordinaryinorganic photoconductors used alone.

Although antimony trisulfide is the preferred inorganic photoconductor,other inorganic photoconductors known in the art, such as cadmiumsulfide, cadmium selenide, and other sulfides and selenides as known inthe art, may be used to practice the invention.

The addition of the organic photoconductors of this invention has beenfound to be particularly advantageous in a camera tube of the typedescribed in Patent No. 3,003,075 to Krieger et al. Krieger particularlydescribes an image orthicon tube, although, as he points out, thisphotosensitive coating is also suitable for a vidicon tube.

The Krieger photosensitive device is designed for sensitivity toinfra-red and contains a composite layer comprising lead oxide, sulfurand oxygen.

It has now been found that the sensitivity of the infrared sensitivedevice of Krieger may be greatly improved by adding thereto an effectiveamount of an organic photoconductor such as, for example, copperphthalocyanine. For example, where a photosensitive target is made bythe successive deposition of layers of lead oxide and sulfur, asdescribed in the Krieger patent, if about 5% of copper phthalocyanine isalso deposited, the sensitivity of the layer, particularly in the nearinfra-red area, is increased without detrimentally atfecting the Whiteburn-in of the target. Such a target has a peak sensitivity at about .9microns, and the sensitivity range runs from about .55 to about 3microns.

In applications in which copper phthalocyanine are used, it has beenfound that proportions of copper phthalocyanine less than about 4 partsby Weight per parts of the inorganic photoconductor have little or noeffect upon sensitivity, lag, or white burn-in resistance of thecoating. Furthermore, it has also been found that proportions higherthan about 10 parts per 100 parts of the inorganic photoconductoractually out down the sensitivity and makes a very grainy picture.

Many modifications and variations in this invention will be apparent tothose skilled in the art, and therefore the invention is not limited tothe embodiments shown and described herein but only as set forth in thefollowing claim.

Iclaim:

1. A target for a photoconductive camera tube comprising a signalelectrode,

a. porous photoconductive layer disposed substantially over one surfaceof the signal electrode,

and a solid photoconductivc layer disposed substantially over saidporous layer,

said porous layer comprising a mixture of antimony trisulfide, freeantimony, and copper phthalocyanine, the amount of copper phthalocyaninebeing from about 4% to about 10% of the total weight of the antimonytrisulfide and the free antimony.

References Cited UNITED STATES PATENTS 2,692,178 10/1954 Grandadam252501 2,889,188 6/1959 Jacobs et al. 3l3-65 2,959,481 11/1960 Kucera252501 X 2,967,254 1/1961 Forgue 3l3-65 2,972,585 2/1961 Beutler 252-5013,136,909 6/1964 Cope 3 l365 3,165,420 1/1965 Tomanek et al. 961.5 X3,197,307 7/1965 Blake et al. 252- 501 X 3,253,914 5/1966 Schaum et a1252-501 X 3,287,120 11/1966 Hoegl 961.6

FOREIGN PATENTS 623,892 7/1961 Canada.

ROBERT SEGAL, Primary Examiner.

JAMES W. LAWRENCE, Examiner.

